As a related vehicle seat, there is a seat designed such that a lateral part of the seat on a vehicle body side is supported by a hinge mechanism so as to be rotatable between a position at which the seat is raised and a position at which one can sit thereon, and a lock device is provided to a leg of the seat given on the opposite side thereof from the vehicle body, i.e. the cabin side (see Patent Literature 1) . The lock device is formed of front lock means and rear lock means which are capable of engaging with front and rear engagement members fixed to a floor panel. While the rear lock means is in an engaged state, the front lock means is in a disengaged state, but the front lock means can be moved into engagement with the front engagement member by given lifting load applied to the seat cushion in a rear collision.